How do I test my interface functions?

Third day in Go so pardon if this is a newb question ;). I am creating a simple calculator that will eventually have many different tasks: addition, subtraction, multiplication, etc, etc...Each of the tasks will have 2 functions: First and Second.

package main

import (
	"github.com/mytestproj/calculator"
)

type Calc interface {
	First(x int) int
	Second(x int) int
}

func main() {
	x := 16
	var i Calc
	a := calculator.Add{}
	i = a
	i.First(x)
	i.Second(x)
}

I currently have everything organized as:

github.com/
   mytestproj/
      calculator/
         addition.go
         addition_test.go
         subtraction.go
         subtraction_test.go
   main/
      main.go

In addition.go I have:

package calculator

type Add struct{}

func BasicAddition(x int) int {  // this won't be in the final release
	return x + 2
}

func (h Add) First(x int) int {
	x += 5
	return x
}

func (h Add) Second(x int) int {
	x += 10
	return x
}

And in addition_test.go I have:

package calculator

import "testing"

func TestBasicAddition(t *testing.T) {
	x := 30
	if y := BasicAddition(x); y != 32 {
		t.Errorf("Mine is %v", y)
	}
}

func TestFirst(t *testing.T) {
	x := 10
	if y := First(x); y != 15 {
		t.Errorf("First is %v", y)
	}
}

When I run my test I get an error:

# github.com/mytestproj/calculator
./addition_test.go:15: undefined: First
FAIL	github.com/mytestproj/calculator [build failed]

My question: How do I test "First?"

If I remove the test for First completely then the tests run fine and it passes.

Secondary question: The idea is to build a calculator app with many different functions. If there's a better way to organize the code then please let me know.

First is a method on type Add. For example,

a := Add{}
a.First(x)

addition.go

package calculator

type Add struct{}

func BasicAddition(x int) int { // this won't be in the final release
	return x + 2
}

func (h Add) First(x int) int {
	x += 5
	return x
}

func (h Add) Second(x int) int {
	x += 10
	return x
}

addition_test.go

package calculator

import "testing"

func TestBasicAddition(t *testing.T) {
	x := 30
	if y := BasicAddition(x); y != 32 {
		t.Errorf("Mine is %v", y)
	}
}

func TestFirst(t *testing.T) {
	x := 10
	a := Add{}
	if y := a.First(x); y != 15 {
		t.Errorf("First is %v", y)
	}
}

Output:

$ go test -v
=== RUN TestBasicAddition
--- PASS: TestBasicAddition (0.00s)
=== RUN TestFirst
--- PASS: TestFirst (0.00s)
PASS
ok  	so/calculator	0.002s
$